Lunar and Planetary Science XXXVIII (2007)
1678.pdf
RECENT GULLY ACTIVITY ON MARS: CLUES FROM LATE-STAGE WATER FLOW IN GULLY
SYSTEMS AND CHANNELS IN THE ANTARCTIC DRY VALLEYS. J. L. Dickson1, J. W. Head1, D. R. Marchant2, G. A. Morgan1, and J. S. Levy1; 1Dept. Geol. Sci., Brown Univ., Providence, RI 02912 USA
(james_dickson@brown.edu), 2Dept. Earth Sci., Boston Univ., Boston MA 02215 USA (marchant@bu.edu).
Introduction: One of the major surprises of the
Mars Global Surveyor mission was the discovery of gullies, a class of unusually young features consisting of
an alcove, a channel and a fan [1-2]. These features
were interpreted to have originated through the flow of
liquid water originating from groundwater discharge
[1,2], although alternate interpretations have been presented [see 3]. Uncertainty as to the possibility of surface water flow under current martian conditions (below
the triple point of H2O) and evidence that conditions
might have been more conducive to melting during
higher obliquity conditions in the past, led to the idea
that these features might be relict. Recent repeat observations of gullies, however, are interpreted to mean that
at least some gullies are currently active [4], and this
has intensified the discussion concerning their formation
mechanisms and age.
In order to look for changes since 2000 that might
indicate the presence of fluid flow in gully channels,
Malin et al. [4] repeatedly imaged thousands of gullys at
hundreds of different sites; they ultimately found only
two sites at which changes could be documented. These
changes included the appearance of light-toned flows
that formed at the two locations sometime between the
two imaging sequences (8/99-2/04 and 12/01-5/05). On
the wall of the crater at the Terra Serenum site a distinct
light-toned flow appeared in the channel. On the southern wall of a crater at Centauri Montes, light-toned material flowed down the slope and formed a deposit. The
two new deposits have similar apparent brightness, and
relatively long, extended digitate distal and marginal
branches; they flow down 20-30o slopes, have relatively
low relief and divert around obstacles [4]. These characteristics suggest that the material moved slowly, thinned
while flowing, and branched easily [4]. Malin et al. [4]
interpreted these characteristics to mean that the observed deposits were formed by flow of fluidized material through the gullies to their aprons by release of
groundwater from underground aquifers "initiated and
fed by the collapse of an ice-impregnated rock dam creating a brief, low-volume debris flow initially charged
with liquid but in which ongoing freezing at both the top
and bottom surfaces, bed infiltration, and incorporation
of slope sediment and debris increases viscosity, which
inhibits downslope and runout motion" [4]. Because
the light tone has lasted more than a martian year in an
environment where water ice is unstable, Malin et al. [4]
suggest that the brightness may reflect "replenishment
of surface frost by exhalation, elutriation of fine-grained
sediment, or precipitation of salts" [4]. Malin et al. [4]
conclude that water flowed on the surface of Mars during the last decade and that it was released from an underground aquifer source.
Terrestrial analogs to martian environments may
provide insight into the processes operating on Mars,
and the origin and life cycle of gullies. Here we report
on the results of ongoing field studies of gullies in the
Antarctic Dry Valleys (ADV), a hyperarid cold polar
desert analog for Mars [7]. We address the questions:
What is the life cycle of gullies and what processes are
responsible for their later stages? How does the ADV
insight help to evaluate and understand the evidence [7]
for current gully activity on Mars?
Fig. 1. Perspective view of ADV South Fork gully systems;
note snow in gullies and marginal and distal hyporheic zone
(dark). Person provides scale.
Nature and Life Cycle of Gullies in the ADV:
Streams and gullies in the ADV form from surface topdown melting of snow and ice due to enhanced seasonal
solar insolation [7-8] and albedo-induced melting, in
contrast to underground aquifer [1-2] or subsurface saline spring sources [5-6]. Observed sources for ADV
streams and gullies include: 1) portions of cold-based
glacier surfaces and fronts [8-9], and 2) annual and
perennial snow banks and patches in alcoves and
channels [7-8]. Many ADV gullies display the major
geomorphic components of Mars gullies (alcove,
channel, fan) and commonly occur on equator-facing
slopes. Streams flow during austral summer, for less
than 20% of the year, some only for a few days; they
show considerable daily, intra-seasonal and inter-annual
variation in flow behavior depending on insolation and
air temperature [8]. About half of the stream channels
observed in the ADV are currently active; many others
date from earlier times [8] and thus may be in the
waning stages of formation, or currently inactive. The
range of stream, channel and gully morphologies
observed in the different microenvironments in the
ADV [7-8] suggests that some gullies are in the waning
stages of their evolution; once a gully is formed it can
remain active at a lower level as long as its topography
can capture sufficient winter snow, and the summer
temperatures are sufficiently high to cause melting of
accumulated snow and ice.
Lunar and Planetary Science XXXVIII (2007)
Anatomy of Late Stage Activity in ADV Gully
Channels from Melting of Winter Snowpack Residue: Analysis of gullies in the 2006-2007 austral summer field season [10-12] provided a basis to assess late
stage activity and associated processes. On the equatorfacing slopes of the South Fork of Upper Wright Valley,
the bottom and sides of gully channels contained elongate accumulations of snow and ice up to 1-2 meters
thick and a few meters to over 100 m long (Fig. 1).
Careful monitoring of these snow patches during November-January showed that they rapidly shrank due to
sublimation and melting, breaking up into a series of
smaller patches that ultimately disappeared. During
sunny days, melting associated with these separated
patches resulted in flow of water down the channels.
Stream flow velocities were more rapid than flow advance rates due to the fact that as the flow advanced, it
soaked into the ground creating the marginal and distal
parts of the hyporheic zones. Advance rates were clearly
moderated by insolation and air temperature: clear,
warm calm days were characterized by more rapid advance, cloudy days by much less rapid advance, and
mixed sunny/cloudy days by multiple stages of advance
and retreat. When the Sun was relatively low in the sky
(evening and night) the melting decreased, the water
flow began to retreat (soaking into the hyporheic zone)
and the upper part of the water in the channel froze,
forming a sheet of white ice, and sequestering meltwater
for the night below the ice behind a distal seal. When
heating and melting resumed, the seal breached, and water advanced rapidly in the areas of the just previously
formed hyporheic zone and then slowed again when
reaching parts of the channel floor not yet wetted this
season. This process is repeated daily until advancing
flows from individual snow patches overtake another
snow patch; at this point, due to encountering an established hyporheic zone, channel flow increases in velocity and flux, rapidly overtakes the next advancing front,
and causes an immediate increase in the advance rate.
Maximum advance for one gully/channel system occurred on an almost continuously sunny day during
which time at least six of the major snow patches were
interconnected by flow. Ultimately in some gullies/channels, melting and sublimation exhausted the
supply of surface water contributions, and surface flow
dwindled and ceased, with the upper few cm of the
channel visibly drying. Excavation showed that the hyporheic zone was declining by surface evaporation, and
subsurface drainage; the substrate in the hyporheic zone
below the formerly active channels were damp down to
the top of the ice table (about 15-25 cm), and in some
cases the lower few cm were saturated with water flowing along the top of the ice table to create a distal zone
of surface dampening.
Summary: 1) A wide range of gully activity occurs
in the ADV and variations are seen in both space (microenvironments [7]) and time [8]. Less than half of the
1678.pdf
ADV gullies are currently active, and many of those,
such as the ones described here, are active for only a
short part of the austral summer and then only locally in
parts of the gully; 2) Gully water-flow activity can extend well beyond the period of major topographic formation of the gully; 3) A major mechanism for the extended lifetime is topographic trapping of wind-blow
snow in the austral winter and its heating and melting in
the austral summer; 4) Gully water-flow activity can
continue as long as snow is deposited there and seasonal
conditions are conducive to the melting and flow of water (insolation-induced melting); the presence of lowalbedo materials can preferentially prolong this activity
(albedo-induced melting); 5) Bright deposits can be
produced in the gullies by formation of ice (channel
floor ad distal zone) and deposition of salts (edge of
channel in the hyporheic zone); water flow continues
under the ice cover.
Application to Mars and Recent Activity: 1) Activity was detected in only a small number of gullies on
Mars (2 out of thousands studied [4]) over a multi-year
period; the low level of current activity of the Mars gullies [4] suggests that many of them may be in later
stages of evolution, subsequent to the main period of
channel formation; 2) Deposits and activity in the two
Mars examples are concentrated in the channel section
of the gullies and in their distal regions (the bright digitate deposits) [4]; this is similar to the location and
characteristics of the deposits formed in the ADV from
melting of sequestered wind-blown snowpack, and water flow into the distal fan portion of the gully system;
3) The deposits on Mars are bright relative to their surroundings [4]; in the ADV cases studied, diurnal freezing of the channel formed bright ice, and variations in
wetting and evaporation of the hyporheic zone deposits
bright salts; 4) Recent gully activity on Mars occurs on
crater interior walls [4]; gully presence and activity in
the ADV is a function of microenvironment [7]. On the
basis of these observations, we suggest that recent gully
activity reported on Mars [4] may be due to late-stage
activity caused by seasonal melting of windblown snow
trapped and accumulated within the channel in the winter, similar to a mechanism suggested for gully formation as a whole [13].
References: [1] M. Malin and K. Edgett, Science, 288, 2330,
2000; [2] M. Malin and K. Edgett, JGR, 106, 23429, 2001; [3]
MEPAG SR-SAG, Astrobiology, 6, 677, 2006; [4] M. Malin et
al., Science, 314, 1573, 2006; [5] J. Heldmann et al., JGR,
110, E05004, 2005; [6] J. Heldmann et al., Arc., Ant. Alpine
Res., 37, 127, 2005; [7] D. Marchant and J. Head, Icarus, in
revision, 2007; [8] D. McKnight et al., BioScience, 49, 985,
1999; [9] A. Fountain et al., BioScience, 49, 961, 1999; [10]
G. Morgan et al., LPSC 38, 1656, 2007; [11] J. Head et al.,
LPSC 38, #1617, 2007; [12] J. Levy et al., LPSC 38, this volume, 2007; [13] P. Christensen, Nature, 422, 45, 2003.